Method for fabricating semiconductor substrate, semiconductor substrate, and semiconductor device

ABSTRACT

In a method for fabricating a semiconductor substrate according to an embodiment, an SiC substrate is formed by vapor growth and C (carbon) is introduced into the surface of the SiC substrate to form an n-type SiC layer on the SiC substrate by an epitaxial growth method.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2014-057283, filed on Mar. 19, 2014, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a method forfabricating a semiconductor substrate, a semiconductor substrate, and asemiconductor device.

BACKGROUND

SiC (silicon carbide) is expected as a material for next-generationsemiconductor devices. Compared with Si (silicon), SiC has superiorphysical properties such as three times the band gap, about 10 times thebreakdown field strength, and about three times the thermalconductivity. By making use of such properties, high-breakdown andlow-loss semiconductor devices capable of operating at high temperaturecan be realized.

On the other hand, n-type SiC has a disadvantage of a short life time ofminority carriers. With a short life time of minority carriers, it isdifficult to reduce an on resistance of bipolar devices using n-type SiCas a drift layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a semiconductor substrateaccording to a first embodiment;

FIGS. 2A and 2B are diagrams illustrating functions of the semiconductorsubstrate according to the first embodiment;

FIG. 3 is a schematic sectional view of a semiconductor substrateaccording to a modification of the first embodiment;

FIG. 4 is a schematic sectional view of a semiconductor substrateaccording to a second embodiment;

FIG. 5 is a schematic sectional view of a semiconductor substrateaccording to a modification of the second embodiment;

FIG. 6 is a schematic sectional view of a semiconductor substrateaccording to a third embodiment;

FIG. 7 is a schematic sectional view of the semiconductor substratewhile being fabricated by a method for fabricating the semiconductorsubstrate according to the third embodiment; and

FIG. 8 is a schematic sectional view of a semiconductor device accordingto a fourth embodiment.

DETAILED DESCRIPTION

In a method for fabricating a semiconductor substrate according to anembodiment, an SiC substrate is formed by vapor growth and C (carbon) isintroduced into the surface of the SiC substrate to form an n-type SiClayer on the SiC substrate by an epitaxial growth method.

Hereinafter, embodiments will be described with reference to thedrawings. In the description that follows, the same reference numeralsare attached to the same members and so on and a description of a memberor the like described once is omitted when appropriate.

Also in the description that follows, n⁺, n, and n⁻ and p⁺, p, and p⁻indicate the relative level of impurity concentration of each conductivetype. That is, n⁺ indicates that the n-type impurity concentrationthereof is higher than n and n⁻ indicates that the n-type impurityconcentration thereof is lower than n. Also, p⁺ indicates that thep-type impurity concentration thereof is higher than p and p⁻ indicatesthat the p-type impurity concentration thereof is lower than p.Incidentally, the n⁺ type and the n⁻ type may simply be written as the ntype and the p⁺ type and the p⁻ type may simply be written as the ptype.

FIRST EMBODIMENT

A semiconductor substrate according to the present embodiment includesan SiC substrate having a region in which the Z_(1/2) level densitymeasured by DLTS (Deep Level Transient Spectroscopy) is 1×10 ¹¹ cm⁻³ orless.

FIG. 1 is a schematic sectional view of a semiconductor substrateaccording to the present embodiment. A semiconductor substrate 100 is anSiC substrate 10. The SiC substrate 10 includes a high carbonconcentration region (region) 11 on the surface thereof.

The SiC substrate 10 is an n⁺ single-crystal SiC substrate. For example,the SiC substrate 10 is a substrate of 4H—SiC whose surface is inclinedat an off angle of 0.2 to 10 degrees with respect to the {0001} plane.

The n-type impurity contained in the SiC substrate 10 is, for example, N(nitrogen) and the impurity concentration of the n-type impurity is, forexample, 5×10¹⁸ cm⁻³ or more and 1×10²⁰ cm⁻³ or less. The thickness ofthe SiC substrate 10 is, for example, 100 μm or more and 400 μm or less.

The high carbon concentration region 11 has, when compared with otherregions of the SiC substrate 10, a high interstitial carbonconcentration. The interstitial carbon concentration has a negativecorrelation with a carbon vacancy concentration. Therefore, the highcarbon concentration region 11 has, when compared with other regions ofthe SiC substrate 10, a low carbon vacancy concentration.

The carbon vacancy concentration and the Z_(1/2) level density measuredby DLTS (Deep Level Transient Spectroscopy) are positively correlated.The Z_(1/2) level density in the high carbon concentration region 11measured by DLTS is 1×10¹¹ cm⁻³ or less.

The semiconductor substrate 100 according to the present embodiment is,for example, a semiconductor substrate used for fabricating asemiconductor substrate applicable to a PIN diode as a bipolar device.

The n-type SiC has a disadvantage that when compared with, for example,n-type Si, the life time of minority carriers (holes) is short. If thelife time of minority carriers is short, in the case of, for example, abipolar device in which the n-type SiC is used for the drift layer,conductivity modulation is insufficient and it is difficult to reducethe on resistance of the device.

As a factor of the short life time of minority carriers, carbon vacancyin the n-type SiC can be considered. That is, the carbon vacancy acts asa killer center of holes, reducing the life time of holes. Therefore,the life time of holes can be considered to be prolonged by reducing thecarbon vacancy in the n-type SiC.

FIGS. 2A and 2B are diagrams illustrating functions of the semiconductorsubstrate according to the present embodiment. FIG. 2A is a case when ann-type SiC layer is formed on an SiC substrate having no high carbonconcentration region by the epitaxial growth method and FIG. 2B is acase when an n-type SiC layer is formed on an SiC substrate having ahigh carbon concentration region by the epitaxial growth method. Foreach case, the distribution of interstitial carbon concentration in thethickness direction of an SiC substrate is separately shown for a casewhen thermal diffusion during epitaxial growth is not considered (nothermal diffusion) and a case when thermal diffusion is considered (withthermal diffusion).

An n-type SiC layer used for the drift layer of a bipolar device isgenerally formed on a single-crystal SiC substrate fabricated byundergoing a high-temperature process of 1700° C. or higher such as thesublimation method, high-temperature CVD (Chemical Vapor Deposition)method or the like by using the epitaxial growth method. Theinterstitial carbon concentration depends on the formation temperatureof SiC and an SiC substrate fabricated at high temperature has a lowinterstitial carbon concentration. In other words, the interstitialcarbon concentration depends on the formation temperature of SiC and anSiC substrate fabricated at high temperature has a high carbon vacancyconcentration. In general, the epitaxial growth temperature of an n-typeSiC layer is lower than the formation temperature of the SiC substrate.

A solid line (no diffusion) in FIG. 2A indicates the distribution ofinterstitial carbon concentration when it is assumed that interstitialcarbon does not diffuse by heat treatment during formation of the n-typeSiC layer or thereafter. The interstitial carbon concentration of then-type SiC layer formed at a lower temperature than the SiC substratebecomes higher.

In reality, however, as indicated by a dotted line (with diffusion) inFIG. 2A, interstitial carbon is diffused by heat treatment duringformation of the n-type SiC layer or thereafter to generate aconcentration gradient. At this point, carbon vacancy is similarlydiffused to generate a concentration gradient.

Interstitial carbon in the n-type SiC layer is diffused toward the SiCsubstrate where the interstitial carbon concentration is low duringformation of the n-type SiC layer and the interstitial carbonconcentration in the n-type SiC layer is decreased. Particularly theinterstitial carbon concentration on the side of the SiC substrate ofthe n-type SiC layer is decreased. In other words, the carbon vacancy onthe side of the SiC substrate of the n-type SiC layer is increased.

On the other hand, in the present embodiment, as shown in FIG. 2B, ahigh carbon concentration region where the interstitial carbonconcentration is high is present on the surface of the SiC substrate.Thus, interstitial carbon in the high carbon concentration region isdiffused toward the side of the n-type SiC layer during formation of then-type SiC layer and the interstitial carbon concentration in the n-typeSiC layer is increased. Particularly the interstitial carbonconcentration on the side of the SiC substrate of the n-type SiC layeris increased. In other words, the carbon vacancy on the side of the SiCsubstrate of the n-type SiC layer is decreased.

Thus, if an n-type SiC layer is formed by the epitaxial growth methodusing the semiconductor substrate 100 according to the presentembodiment, the carbon vacancy concentration in the n-type SiC layer canbe reduced. Therefore, the life time of minority carriers in the n-typeSiC layer, that is, holes can be improved.

(Modification)

FIG. 3 is a schematic sectional view of a semiconductor substrateaccording to a modification of the present embodiment. A semiconductorsubstrate 200 is different from the semiconductor substrate 100 in thefirst embodiment in that the SiC substrate 10 is a p⁺ typesingle-crystal SiC substrate.

The p-type impurity in the SiC substrate 10 is, for example, Al(aluminum) and the impurity concentration of the p-type impurity is, forexample, 5×10¹⁸ cm⁻³ or more and 1×10²⁰ cm⁻³ or less.

The semiconductor substrate 200 according to the present embodiment is,for example, a semiconductor substrate used for fabricating asemiconductor substrate applicable to vertical IGBT (Insulated GateBipolar Transistor) as a bipolar device.

If an n-type SiC layer is formed by the epitaxial growth method usingthe semiconductor substrate 200 according to the present embodiment,like in the first embodiment, the carbon vacancy concentration in then-type SiC layer can be reduced. Therefore, the life time of minoritycarriers in the n-type SiC layer can be improved.

SECOND EMBODIMENT

In a method for fabricating a semiconductor substrate according to thepresent embodiment, an SiC substrate is grown by the vapor growth and C(carbon) is introduced into the SiC substrate to form an n-type SiClayer on the SiC substrate by the epitaxial growth method. The methodfor fabricating a semiconductor substrate according to the presentembodiment is a fabricating method using a semiconductor substrate inthe first embodiment. Therefore, the description of content overlappingwith the content in the first embodiment is omitted. The semiconductorsubstrate according to the present embodiment is a semiconductorsubstrate fabricated by using the above fabricating method.

FIG. 4 is a schematic sectional view of a semiconductor substrateaccording to the present embodiment. A semiconductor substrate 300includes the SiC substrate 10 and an n-type SiC layer 12 on the SiCsubstrate 10. The high carbon concentration region 11 is provided in aportion of the SiC substrate 10 in contact with the n-type SiC layer 12.

The SiC substrate 10 is an n⁺ single-crystal SiC substrate. For example,the SiC substrate 10 is a substrate of 4H—SiC whose surface is inclinedat an off angle of 0.2 to 10 degrees with respect to the {0001} plane.

The n-type impurity contained in the SiC substrate 10 is, for example, N(nitrogen) and the impurity concentration of the n-type impurity is, forexample, 5×10¹⁸ cm⁻³ or more and 1×10²⁰ cm⁻³ or less. The thickness ofthe SiC substrate 10 is, for example, 100 μm or more and 400 μm or less.

The high carbon concentration region 11 has, when compared with otherregions of the SiC substrate 10, a high interstitial carbonconcentration. The interstitial carbon concentration has a negativecorrelation with a carbon vacancy concentration. Therefore, the highcarbon concentration region 11 has, when compared with other regions ofthe SiC substrate 10, a low carbon vacancy concentration.

The carbon vacancy concentration and the Z_(1/2) level density measuredby DLTS (Deep Level Transient Spectroscopy) are positively correlated.The Z_(1/2) level density in the high carbon concentration region 11measured by DLTS is 1×10¹¹ cm⁻³ or less.

The n-type SiC layer 12 contains, for example, N (nitrogen) as an n-typeimpurity. The impurity concentration of the n-type impurity of then-type SiC layer 12 is lower than that of the n-type impurity of the SiCsubstrate 10. The impurity concentration of the n-type impurity of then-type SiC layer 12 is, for example, 1×10¹⁵ cm⁻³ or more and 5×10¹⁶ cm⁻³or less.

The thickness of the n-type SiC layer 12 is, for example, 5 μm or moreand 200 μm or less. From the viewpoint of using for fabricatinghigh-breakdown devices, the thickness of the n-type SiC layer 12 isdesirably 50 μm or more and more desirably 100 μm.

The semiconductor substrate 300 according to the present embodiment is,for example, a semiconductor substrate used for fabricating a PIN diodeas a bipolar device.

Next, the method for fabricating a semiconductor substrate according tothe present embodiment will be described with reference to FIG. 4.

First, the SiC substrate 10 is formed by the vapor growth. The vaporgrowth is, for example, the sublimation method or the high-temperatureCVD method. From the viewpoint of improving throughput of fabrication byincreasing the growth speed, the formation temperature of the SiCsubstrate 10 is desirably 1700° C. or higher, more desirably 1800° C. orhigher, and most desirably 1900° C. or higher. If the formationtemperature of the SiC substrate 10 is high, the concentration ofinterstitial carbon in the SiC substrate 10 decreases and theconcentration of carbon vacancy increases.

Next, C (carbon) is introduced into the surface of the SiC substrate 10to form the high carbon concentration region 11. The high carbonconcentration region 11 is formed by ion implantation of C (carbon).

From the viewpoint of improving crystallinity of the n-type SiC layer 12formed later by the epitaxial growth method, it is desirable to reducedamage by ion implantation on the surface of the SiC substrate 10 afterthe ion implantation as much as possible. From the above viewpoint, itis desirable to control a projected range (Rp) during ion implantationof C (carbon). That is, it is desirable to set acceleration energy suchthat the position of Rp±3σ where the concentration of ions is two ordersof magnitude lower than the peak concentration is on the inner side fromthe surface of the SiC substrate 10. It is more desirable to setacceleration energy such that the position of Rp±4.8σ where theconcentration of ions is five orders of magnitude lower than the peakconcentration is on the inner side from the surface of the SiC substrate10.

From a similar viewpoint, it is desirable to perform ion implantation ina state in which the surface of the SiC substrate 10 is exposed withoutan oxide film such as a through film being provided on the surface ofthe SiC substrate 10 so that the projected range (Rp) during ionimplantation of (carbon) is positioned in a deeper position of thesubstrate with the same amount of acceleration energy.

Next, the n-type SiC layer 12 is formed on the SiC substrate 10 by theepitaxial growth method. The n-type SiC layer 12 is formed on thesurface on the side of the high carbon concentration region 11 of theSiC substrate 10.

From the viewpoint of reducing carbon vacancy, the formation temperaturewhen the n-type SiC layer 12 is formed is desirably lower than that ofthe SiC substrate 10. The formation temperature is, for example, 1550°C. or higher and 1650° C. or lower.

The material gas of Si (silicon) when the n-type SiC layer 12 is formedis, for example, mono-silane (SiH₄) using a hydrogen gas (H₂) as acarrier gas. Also, the material gas of C (carbon) is formed is, forexample, propane (C₃H₈) using a hydrogen gas as a carrier gas. Also, thematerial gas of N (nitrogen) as an n-type impurity is formed is, forexample, a nitrogen gas (N₂) diluted by a hydrogen gas.

The thickness of the n-type SiC layer 12 to be formed is, for example,50 μm.

According to the method for fabricating a semiconductor substrate in thepresent embodiment, the carbon vacancy concentration of the n-type SiClayer 12 is reduced by introducing C (carbon) into the surface of theSiC substrate 10 to form the high carbon concentration region 11.Therefore, the semiconductor substrate 300 in which the life time ofminority carriers is improved can be fabricated. Then, by using thesemiconductor substrate 300 according to the present embodiment, abipolar device that reduces on resistance can be fabricated.

The method for fabricating a semiconductor substrate according to thepresent embodiment is effective in reducing carbon vacancy whenparticularly a thick n-type SiC layer for high-breakdown devices isformed because interstitial carbon is introduced into the n-type SiClayer from the side of the SiC substrate 10.

The method for introducing C (carbon) into the surface of the SiCsubstrate 10 may be a method that diffuses C (carbon) from a carboncontaining film into the SiC substrate 10 by forming the carboncontaining film on the surface of the SiC substrate 10 and heat-treatingthe carbon containing film. As the carbon containing film, for example,a carbon film formed by a sputtering process or a film carbonized byheat-treating a photo resist can be applied.

(Modification)

FIG. 5 is a schematic sectional view of a semiconductor substrateaccording to a modification of the present embodiment. A semiconductorsubstrate 400 is different from the semiconductor substrate 200 in thesecond embodiment in that the SiC substrate 10 is a p⁺ typesingle-crystal SiC substrate.

The p-type impurity in the SiC substrate 10 is, for example, Al(aluminum) and the impurity concentration of the p-type impurity is, forexample, 5×10¹⁸ cm⁻³ or more and 1×10²⁰ cm⁻³ or less. The impurityconcentration of the n-type impurity of the n-type SiC layer 12 is lowerthan that of a p-type impurity of the SiC substrate 10.

The semiconductor substrate 400 according to the present embodiment is,for example, a semiconductor substrate used for fabricating a verticalIGBT as a bipolar device.

THIRD EMBODIMENT

The method for fabricating a semiconductor substrate according to thepresent embodiment is the same as that in the second embodiment exceptthat C (carbon) is selectively introduced into the surface of an SiCsubstrate. In addition, a semiconductor substrate according to thepresent embodiment is the same as that in the second embodiment exceptthat the above region is selectively provided on the surface of an SiCsubstrate. Therefore, the description of content overlapping with thecontent in the second embodiment is omitted.

FIG. 6 is a schematic sectional view of a semiconductor substrateaccording to the present embodiment. A semiconductor substrate 500includes the SiC substrate 10 and the n-type SiC layer 12 on the SiCsubstrate 10. The high carbon concentration region 11 is provided in aportion of the SiC substrate 10 in contact with the n-type SiC layer 12.

The high carbon concentration region 11 is selectively provided on theside of the SiC substrate 10 of the interface between the SiC substrate10 and the n-type SiC layer 12. In other words, the n-type SiC layer 12is in contact with other portions other than the high carbonconcentration region 11 of the SiC substrate 10.

Next, the method for fabricating a semiconductor substrate according tothe present embodiment will be described with reference to FIGS. 6 and7. FIG. 7 is a schematic sectional view of the semiconductor substratewhile being fabricated by the method for fabricating the semiconductorsubstrate according to the present embodiment.

When C (carbon) is introduced into the surface of the SiC substrate 10by ion implantation, in contrast to the second embodiment, the ionimplantation is performed into the surface of the SiC substrate 10 byusing a patterned mask material 33 as a mask. The mask material 33 is,for example, a photo resist.

After the mask material being removed after the ion implantation, then-type SiC layer 12 is formed.

According to the method for fabricating a semiconductor substrate in thepresent embodiment, when the n-type SiC layer 12 is formed by epitaxialgrowth, in addition to the high carbon concentration region 11, a lowcarbon concentration region 13 where the carbon concentration is low andno damage is done by ion implantation is present on the surface of theSiC substrate 10.

The n-type SiC layer 12 superior in crystallinity can be formed bygrowing the n-type SiC layer 12 or the low carbon concentration region13 good in crystallinity as a seed.

Devices superior in characteristics can be fabricated by using thesemiconductor substrate 500 according to the present embodiment.

Incidentally, effects of the present embodiment can also be obtained byusing the method of diffusing C (carbon) from a carbon containing filminto the SiC substrate 10 when C (carbon) is introduced into the surfaceof the SiC substrate 10. It is easier to form the n-type SiC layer 12good in crystallinity on the surface of the SiC substrate 10 in whichthe carbon concentration is low than on the surface of the SiC substrate10 in which the carbon concentration is high. By patterning the carboncontaining film before heat treatment, C (carbon) can selectively beintroduced into the surface of the SiC substrate 10.

FOURTH EMBODIMENT

A semiconductor device according to the present embodiment includes asemiconductor substrate according to the second embodiment. Therefore,the description of content overlapping with the content in the secondembodiment is omitted.

FIG. 8 is a schematic sectional view of a semiconductor device accordingto the present embodiment. A semiconductor device 600 according to thepresent embodiment is a mesa PIN diode.

The PIN diode 600 includes the SiC substrate 10. The SiC substrate 10 isan n⁺ single-crystal SiC substrate. For example, the SiC substrate 10 isa substrate of 4H—SiC whose surface is inclined at an off angle of 0.2to 10 degrees with respect to the {0001} plane.

The n-type SiC layer 12 is formed on the SiC substrate 10. The n-typeSiC layer 12 is an epitaxial growth layer. The n-type SiC layer 12 is adrift layer of the PIN diode 600. The high carbon concentration region11 is provided in a portion of the SiC substrate 10 in contact with then-type SiC layer 12.

The n-type SiC layer 12 contains, for example, N (nitrogen) as an n-typeimpurity. The impurity concentration of the n-type SiC layer 12 is, forexample, 1×10¹⁵ cm⁻³ or more and 5×10¹⁶ cm⁻³ or less. The thickness ofthe n-type SiC layer 12 is, for example, 5 μm or more and 200 μm orless.

A p-type SiC layer 14 containing a p-type impurity is formed on then-type SiC layer 12. The p-type SiC layer 14 is an epitaxial growthlayer.

The p-type SiC layer 14 contains, for example, Al (aluminum) as a p-typeimpurity and the impurity concentration thereof is 1×10¹⁶ cm⁻³ or moreand 1×10²² cm⁻³ or less. The thickness of the p-type SiC layer 14 is,for example, 0.2 μm or more and 3 μm or less.

Then, the PIN diode includes a conductive anode electrode 16electrically connected to the p-type SiC layer 14. The anode electrode16 is formed of, for example, a barrier metal layer 16 a of Ni (nickel)and an metal layer 16 b of Al (aluminum) on the barrier metal layer 16a.

In addition, a conductive cathode electrode 18 is formed on the backside of the SiC substrate 10. The cathode electrode 18 is, for example,Ni (nickel).

The PIN diode 600 is provided with a groove portion 20 provided on bothsides of the anode electrode 16 and reaching the n-type SiC layer 12from the surface of the p-type SiC layer 14. The groove portion 20 isfilled with, for example, an oxide film (not shown). By providing thegroove portion 20, the high-breakdown PIN diode 600 that reduces a leakcurrent can be realized.

In the semiconductor device according to the present embodiment, carbonvacancy in the n-type SiC layer 12 to be a drift layer is reduced.Therefore, the life time of holes in the n-type SiC layer 12 isprolonged and the PIN diode 600 of low on resistance can be realized.

In the above embodiments, a case of 4H—SiC as the crystal structure ofsilicon carbide is taken as an example, but the embodiments can also beapplied to other crystal structures such as 6H—SiC and 3C—SiC.

The embodiments have been described by taking a PIN diode as an exampleof the bipolar device, but the embodiments can also be applied to otherbipolar devices such as IGBT (Insulated Gate Bipolar Transistor) and BJT(Bipolar Junction Transistor) in which an n-type SiC layer is used forthe drift layer.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the method for fabricating thesemiconductor substrate, the semiconductor substrate, and thesemiconductor device described herein may be embodied in a variety ofother forms; furthermore, various omissions, substitutions and changesin the form of the devices and methods described herein may be madewithout departing from the spirit of the inventions. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and spirit of theinventions.

What is claimed is:
 1. A semiconductor substrate comprising an SiCsubstrate having a region where a Z_(1/2) level density measured by DLTS(Deep Level Transient Spectroscopy) is 1×10¹¹ cm⁻³ or less.
 2. Thesemiconductor substrate according to claim 1, further comprising ann-type SiC layer provided on the SiC substrate, the n-type SiC layerhaving an impurity concentration of an n-type impurity lower than animpurity concentration of the n-type impurity and a p-type impurity ofthe SiC substrate.
 3. The semiconductor substrate according to claim 1,wherein the impurity concentration of the n-type or p-type impurity inthe SiC substrate is 5×10¹⁸ cm⁻³ or more.
 4. The semiconductor substrateaccording to claim 1, wherein a thickness of the n-type SiC layer is 50μm or more.
 5. The semiconductor substrate according to claim 1, whereinthe region is selectively provided in the SiC substrate.
 6. Asemiconductor device comprising: an SiC substrate having a region wherea Z_(1/2) level density measured by DLTS (Deep Level TransientSpectroscopy) is 1×10¹¹ cm⁻³ or less and an n-type SiC layer provided onthe SiC substrate, the n-type SiC layer having an impurity concentrationof an n-type impurity lower than an impurity concentration of the n-typeimpurity and a p-type impurity of the SiC substrate.
 7. The deviceaccording to claim 6, wherein the impurity concentration of the n-typeor p-type impurity in the SiC substrate is 5×10¹⁸ cm⁻³ or more.
 8. Thedevice according to claim 6, wherein a thickness of the n-type SiC layeris 50 μm or more.
 9. The device according to claim 6, wherein the regionis selectively provided on a side of the SiC substrate of an interfacebetween the SiC substrate and the n-type SiC layer.